Element Magnesium, Be, Alkaline Earth Metal

About Magnesium

Magnesium bears the same relation to calcium as sodium does to potassium. This relation finds expression not only in the values of the combining weights, but also in the similarities to the other members of the group. This is particularly well seen in the fact that magnesium is of more frequent occurrence than calcium, and that, in its properties, it differs from calcium more than the latter does from the corresponding elements of higher combining weight, strontium and barium.

That an elementary metal is present in the magnesium salt, was regarded by Davy as indubitable from the time that the corresponding fact was recognised in the case of potassium and sodium. Bunsen, however, was the first to prepare the metal itself. He obtained it by the electrolysis of the fused chloride.

The electrolysis can be performed in the lecture by employing fused carnallite as electrolyte. The partition and the prolongation of the crucible are of asbestos mill-board, the cathode is a piece of iron wire, and the anode a thin arc-carbon. The strength of the current is about 3-10 amperes.

Magnesium is now manufactured in very large amount by electrolysis, and is used for various purposes. It is a white, rather tough metal, which keeps fairly well in the air; it is scarcely attacked by cold water, but in boiling water it slowly evolves hydrogen. In dilute acids it very rapidly dissolves, with energetic evolution of hydrogen. It melts at about 750°, and volatilises at a bright white heat.

Heated in the air, magnesium burns with a very bright, white flame, which is largely made use of. For example, instantaneous photographs can be easily taken by magnesium light. For this purpose magnesium is used in powder form, and is either blown through a flame or is mixed with potassium chlorate or perchlorate, sometimes with potassium permanganate, and ignited.

The former method has the advantage that comparatively little metal is required, especially when pure oxygen is used instead of air for blowing it; but it has the disadvantage that the flash lasts for a comparatively long time, about 0.3 second. The second method requires larger quantities of magnesium, but the flash lasts only 0.1 second, or less. The last-mentioned mixture, however, is rather dangerous, as it readily explodes; it ought not, therefore, to be prepared in large quantity.

Magnesium is also used for other purposes where a bright, transient light is required; in such cases the magnesium is generally burned in the form of ribbon in specially constructed lamps. Magnesium is also used for adding to fireworks.

Magnesium is an exceedingly effective reducing agent at a high temperature. Silicon, boron, and most of the other metals can be obtained by heating their oxygen compounds with magnesium powder, the latter thereby passing into its oxide, MgO.

Occurrence of Magnesium

Meteorites, which are samples of extra-terrestrial origin, often contain combined magnesium. It is, after iron, their most important metallic constituent. Magnesium has been spectroscopically discovered in the sun, and in the earth's crust the number of its atoms has been estimated as 1.67 per cent, of the total of elementary atoms.

Magnesium compounds are widely distributed over the earth. Magnesium chloride, MgCl2, and, to a lesser extent, magnesium bromide, MgBr2, and magnesium iodide, MgI2, occur in natural waters - including sea-water. The slightly soluble magnesium fluoride, MgF2, occurs as the mineral sellaite. Sea-water and most mineral springs contain more salts of magnesium than of calcium, but in river-water calcium salts preponderate. Magnesium sulphate is a prevalent magnesium salt in natural water, and Epsom salts, MgSO4.7H2O, derive their name from their occurrence in the famous mineral spring at Epsom. MgSO4.7H2O occurs as the mineral epsomite or, as it is called when found in the Stassfurt deposits, reichardtite.

The Stassfurt deposits had a marine origin, and consist of narrow bands of anhydrous calcium sulphate, interspersed, at fairly regular intervals, through a mass of rock salt. Beds rich in magnesium and potassium salts cap the narrow seams of calcium sulphate. The deposits divide roughly into four regions -

Anhydrite, CaSO4.

Polyhalite, 2CaSO4.MgSO4.KSO4.2H2O, about 60 metres.

Kieserite, MgSO4.H2O, about 30 metres.

Carnallite, MgCl2.KCl4.6H2O, about 23 metres.

These deposits are the most important commercial source of magnesium and its salts. They contain also the following salts - tachhydrite, CaCl2.2MgCl2.12H2O, krugite, KSO4.4CaSO4.MgSO4.2H2O, bischoffite, MgCl2.6H2O, kainite, KCl.MgSO4.3H2O, schonite or picromerite, KSO4.MgSO4.6H2O, leonite, KSO4.MgSO4.4H2O, langbeinite, KSO4.2MgSO4, blodite or astrakanite, Na2SO4.MgSO4.4H2O, loewite, Na2SO4.MgSO4.2H2O, and vanthoffite, 3Na2SO4.MgSO4.

The double sulphate, (NH4)2SO4.MgSO4.6H2O, occurs as boussingaultite.

Many natural waters contain magnesium bicarbonate, Mg(HCO3)2; normal carbonates are represented in many mineral species. MgCO3 occurs as magnesite; breunnerite, (Mg,Fe)CO3, occurs in meteorites. Nesquehonite is MgCO3.3H2O, and dolomite, (Ca,Mg)CO3, is widely distributed. Many mountain ranges consist largely of this important mineral species. Ankerite is (Mg,Fe)CO3.CaCO3.

History of Magnesium

In 1695 Nehemiah Grew, a London physician, published an account of a medicinal salt obtained from the well-known mineral spring at Epsom. "Epsom salts" soon became famous, and are still used in medicine. George and Francis Moult established a factory in 1700 to obtain the salt from a spring at Shooter's Hill, near London. It was soon afterwards discovered by Friedrich Hoffmann in Seidlitz mineral water, and crystallised by Hoy from the mother-liquors of sea-water. The history of magnesium thus began with its sulphate, MgSO4.7H2O.

"Magnesia alba" was discovered, and its medicinal value observed, early in the eighteenth century. Since pyrolusite was then called magnesia nigra, the new substance was apparently named to contrast with it. Valentini in 1707, and Slevogt in 1709, prepared it from saltpetre mother-liquors. Partly because these preparations contained calcium carbonate, magnesia and lime were constantly confused till Black, in 1755, clearly distinguished them. He showed that magnesia alba was a compound of " fixed air " with a peculiar earth, which had a soluble sulphate. When Davy, in 1808, showed that magnesia was the oxide of a metal, he named the metal magnium. Finally, the name magnesium was adopted for the metal in magnesia, and manganese for the metal in pyrolusite. The latter had been known both as magnesium and manganesium.

"Magnesia" is derived from "magnesian stone," an alchemistic designation of the loadstone, and a stone shining like silver, which may have been talc. A confusion of pyrolusite with the loadstone may have led to the terms magnesia alba and magnesia nigra.

The metal was first prepared in mass by Bussy in 1829. He decomposed magnesium chloride with potassium; electrolytic methods are now used in industry.